Homopolymers of L-isomers of alanine, arginine, histidine, lysine, and proline have been oxidized in the presence of ferrous iron and a chelator, generally citrate. Oxidation of the polymers introduces carbonyls which may be detected by reaction with 2,4-dinitrophenylhydrazine or with p-aminobenzoic acid in the presence of sodium cyanoborohydride. Oxidation results in loss of acid precipitability of these homopolymers. When similar oxidations were conducted with insulin beta chain, similar oxidative changes are seen: increased carbonyl content and decreased precipitation with acid. Amino acid analysis of the insulin beta chain suggests that the attack on the peptide's residues must be random.